1. Field of the Invention
The present invention relates to an image forming apparatus that, according to image data, scans a photosensitive member with a light beam to form a latent image on the photosensitive member, develops the latent image on the photosensitive member, and transfers the developed image onto an image bearing member.
2. Description of the Related Art
Recent electrophotographic image forming apparatuses each form a latent image on a photosensitive member with a laser beam emitted according to image data, develop the latent image on the photosensitive member with toner, and then transfer the toner image on the photosensitive member to a recording sheet. Various factors such as an assembly error of an optical system that is provided on an optical path of the laser beam, cause a scanning line of the laser beam irradiated on the photosensitive member to be distorted, for example, to be curved or tilted.
For correcting distortion of a scanning line, Japanese Patent Application Laid-Open No. 2005-304011 discusses a method for correcting image data based on distortion characteristic of the scanning line of the laser beam, and emitting the laser beam according to the image data after correction. In this distortion correction, the main-scanning line is divided into a plurality of regions. Then, the image data in respective regions is shifted in a sub-scanning direction line by line, so that the scanning line of the laser beam corresponding to the image data after correction coincides with the reference line.
Further, in recent years, image forming apparatuses each provided with a plurality of laser light sources have been discussed. In the image forming apparatuses, the plurality of laser light sources can simultaneously form a plurality of scanning lines on the photosensitive member, and to shorten time taken until an image formation is completed (see Japanese Patent Application Laid-Open No. 2003-312041).
If the distortion correction of the scanning line of the laser beam is applied to the image forming apparatus provided with the plurality of laser light sources, the following problems arise. Generally, since positions at which the laser beams from the plurality of laser light sources pass through an optical member are different from one another, distortion characteristics of the scanning lines of the plurality of laser light sources are not the same to one another.
Supposing that divided regions may be individually defined for respective scanning lines, in accordance with the distortion characteristic of each of the scanning lines of the plurality of laser light sources, and image data of the respective divided regions may be shifted line by line in the sub-scanning direction. In such a case, a region where an image is partially missing (between a(1) and b(1) in FIG. 8B) or a region where an image becomes partially thick (between a(2) and b(2) in FIG. 8B) is generated, and image quality is deteriorated.